"Go over the hill" is a common phrase used to describe someone who has declined in physical or mental abilities. However, there are several synonyms that can be used to convey the same sentiment. One of these is "past his/her prime", which implies that the person in question is no longer at their peak or best years. Another synonym is "over the hump" which means that the person has reached a peak in their life and have now passed it. "Past one's sell-by date" is another synonym that suggests the person is no longer relevant or valuable. In conclusion, there are many ways to express the same idea as "go over the hill" and it is up to the speaker to choose the most appropriate one for the situation.